In historical linguistics, Bartsch's law or the Bartsch effect (French: loi de Bartsch, pronounced [lwa də baʁtʃ] or effet de Bartsch) is the name of a sound change that took place in the early history of the langues d'oïl (c. 5th - 6th centuries AD), for example in the development of Old French.

Bartsch's law was a phonetic change affecting the open central vowel [a] in northern Gallo-Romance dialects in the 5th-6th century. This vowel, inherited from Vulgar Latin, underwent fronting and closure in stressed open syllables when preceded by a palatal or palatalized consonant. The result of this process in Old French was the diphthong [ie]:

Latin laxāre /lakˈsaːre/ > Old French laissier [lajˈsier] (modern French laisser "let")

Latin cārum /ˈkaːrum/ > Old French chier [ˈtʃier] (modern French cher, Walloon tchîr "dear")

Note that [ie] is also the outcome of the diphthongization of [ɛ] in stressed, open syllables:

Latin pedem /ˈpedem/ > [ˈpɛdɛ] > [ˈpieðɛ] > Old French pie [ˈpie] (modern French pied "foot")

The chronology of Bartsch's law relative to the more general diphthongization of [a] to [aɛ] (responsible, for example, for the final vowels in mare > mer "sea" or portāre > porter "carry") has not been conclusively established.^[1] According to one view, diphthongization took place first, and Bartsch's law is seen as a further segmentation of the diphthong [aɛ] caused by the preceding palatal/palatalized consonant, followed by simplification of the resulting triphthong:

IPA: [ˈa] > [ˈaɛ̯] > [ˈia̯ɛ̯] > [ˈiɛ̯] > [ˈie̯]

Romanicist notation: á > áę > íaę > íę > íẹ

According to a second view, Bartsch's law affected the simple vowel [a], causing it to change to [e], which then diphthongized to [ie]:

IPA: [a] > [e] > [ˈie̯]

Romanicist notation: a > ẹ > íẹ

Support for the second hypothesis comes the fact that palatal consonants triggered the same change [a] > [e] in unstressed word-initial syllables:^[2]

Latin caballum /kaˈballum/ > [tʃeˈvallo] > Old French cheval [tʃəˈval] "horse"

Subsequent changes have obscured the effects of Bartsch's law in modern French. The accent shifted to the second element of the diphthong [ie], and the first element underwent glide formation:

in IPA: [ˈie̯] > [ˈi̯e] > [je]

in Romanist notation: íẹ > iẹ́ > yẹ

The glide [j] was then lost in most words, either absorbed by the preceding palatal consonant, or eliminated by analogical pressure (e.g. in many verbs of the -er conjugation):^[3]

Old French chier [ˈtʃier] > [ʃjer] > modern French cher [ʃɛr] "dear"

Old French laissier [lajˈsier] > [lajˈsjer] > modern French laisser [lɛˈse] or [leˈse] "let"

The glide was only retained if subsequent nasalization took place, as in Modern French chien [ʃjɛ̃] "dog" (not *chen *[ʃɛ̃] or *[ʃɑ̃]).^[4] Compare with the walloon tchén *[tʃɛ̃]

Consequently, the vowel "e" in these words, which is due to Bartsch's law, is now indistinguishable from the "e" that resulted from the general diphthongization of [a] (as in the words mer "sea", porter "carry", mentioned above). The diphthong [ie] is still visible in the spelling of words like chien "dog" (< canem) and moitié "half" (< Proto-Western Romance [mejˈtate] < Latin medietātem).